Robert W. Robinson. Simplicity of recursively enumerable sets. The journal of symbolic logic, vol. 32 (1967), pp. 162–172. - Robert W. Robinson. Two theorems on hyperhypersimple sets. Transactions of the American Mathematical Society, vol. 128 (1967), pp. 531–538. - A. H. Lachlan. On the lattice of recursively enumerable sets. Transactions of the American Mathematical Society, vol. 130 (1968), pp. 1–37. - A. H. Lachlan. The elementary theory of recursively enumerable sets. Duke mathematical journal, vol. 35 (1968), pp. 123–146.